fscrypto: improved validation when loading inode encryption metadata

- Validate fscrypt_context.format and fscrypt_context.flags.  If
  unrecognized values are set, then the kernel may not know how to
  interpret the encrypted file, so it should fail the operation.

- Validate that AES_256_XTS is used for contents and that AES_256_CTS is
  used for filenames.  It was previously possible for the kernel to
  accept these reversed, though it would have taken manual editing of
  the block device.  This was not intended.

- Fail cleanly rather than BUG()-ing if a file has an unexpected type.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 45 insertions, 23 deletions

diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 1ac263eddc4e..b5374efc4ae3 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -139,6 +139,38 @@ out:
 	return res;
 }
 
+static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
+				 const char **cipher_str_ret, int *keysize_ret)
+{
+	if (S_ISREG(inode->i_mode)) {
+		if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_256_XTS) {
+			*cipher_str_ret = "xts(aes)";
+			*keysize_ret = FS_AES_256_XTS_KEY_SIZE;
+			return 0;
+		}
+		pr_warn_once("fscrypto: unsupported contents encryption mode "
+			     "%d for inode %lu\n",
+			     ci->ci_data_mode, inode->i_ino);
+		return -ENOKEY;
+	}
+
+	if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
+		if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS) {
+			*cipher_str_ret = "cts(cbc(aes))";
+			*keysize_ret = FS_AES_256_CTS_KEY_SIZE;
+			return 0;
+		}
+		pr_warn_once("fscrypto: unsupported filenames encryption mode "
+			     "%d for inode %lu\n",
+			     ci->ci_filename_mode, inode->i_ino);
+		return -ENOKEY;
+	}
+
+	pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
+		     (inode->i_mode & S_IFMT), inode->i_ino);
+	return -ENOKEY;
+}
+
 static void put_crypt_info(struct fscrypt_info *ci)
 {
 	if (!ci)
@@ -155,8 +187,8 @@ int get_crypt_info(struct inode *inode)
 	struct fscrypt_context ctx;
 	struct crypto_skcipher *ctfm;
 	const char *cipher_str;
+	int keysize;
 	u8 raw_key[FS_MAX_KEY_SIZE];
-	u8 mode;
 	int res;
 
 	res = fscrypt_initialize();
@@ -179,13 +211,19 @@ retry:
 	if (res < 0) {
 		if (!fscrypt_dummy_context_enabled(inode))
 			return res;
+		ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
 		ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS;
 		ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS;
 		ctx.flags = 0;
 	} else if (res != sizeof(ctx)) {
 		return -EINVAL;
 	}
-	res = 0;
+
+	if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1)
+		return -EINVAL;
+
+	if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
+		return -EINVAL;
 
 	crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
 	if (!crypt_info)
@@ -198,27 +236,11 @@ retry:
 	crypt_info->ci_keyring_key = NULL;
 	memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
 				sizeof(crypt_info->ci_master_key));
-	if (S_ISREG(inode->i_mode))
-		mode = crypt_info->ci_data_mode;
-	else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
-		mode = crypt_info->ci_filename_mode;
-	else
-		BUG();
-
-	switch (mode) {
-	case FS_ENCRYPTION_MODE_AES_256_XTS:
-		cipher_str = "xts(aes)";
-		break;
-	case FS_ENCRYPTION_MODE_AES_256_CTS:
-		cipher_str = "cts(cbc(aes))";
-		break;
-	default:
-		printk_once(KERN_WARNING
-			    "%s: unsupported key mode %d (ino %u)\n",
-			    __func__, mode, (unsigned) inode->i_ino);
-		res = -ENOKEY;
+
+	res = determine_cipher_type(crypt_info, inode, &cipher_str, &keysize);
+	if (res)
 		goto out;
-	}
+
 	if (fscrypt_dummy_context_enabled(inode)) {
 		memset(raw_key, 0x42, FS_AES_256_XTS_KEY_SIZE);
 		goto got_key;
@@ -253,7 +275,7 @@ got_key:
 	crypt_info->ci_ctfm = ctfm;
 	crypto_skcipher_clear_flags(ctfm, ~0);
 	crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
-	res = crypto_skcipher_setkey(ctfm, raw_key, fscrypt_key_size(mode));
+	res = crypto_skcipher_setkey(ctfm, raw_key, keysize);
 	if (res)
 		goto out;